The present invention is directed to the manufacture of filter media, typically but not necessarily by paper making processes. Felted nonwoven materials are widely used in the manufacture of filter media, such as automative oil and air and fuel filters, for example. Such filters are presently manufactured in large volume by conventional, wet paper-making processes, although air-laying is also a viable and potentially preferable manufacturing procedure. Ideal materials for filter media are bulky, porous, strong, stiff, resistant to heat and chemical degradation, and insensitive to water. The media also should have a desired average pore size for its intended function. These various desirable attributes are controlled by varying the fiber furnish, the treatment of the fibers prior to forming, the manner in which the web is formed, and the post treatment of the formed web. The post treatment may include any or all of the following: pressing, drying, binder addition, and corrugation.
Physical characteristics of the fiber are of course highly significant. In this respect, experience indicates that ideal fibers should be relatively strong and stiff. They should possess some degree of curl, yet be relatively non-bonding. Currently, chemical pulps are widely utilized in the production of fibrous filter media, because the chemical pulps have a superior fiber form to conventional ground wood pulps. As compared to the typical mechanical pulps, chemical pulps are characterized by a greater fiber length, higher individual fiber strength and stiffness, and a relatively large degree of fiber bonding when made into paper. By contrast, conventional ground wood pulps are very short, leading to a filter sheet which lacks desired strength and porosity, among other things.
Unfortunately, the most desirable grades of chemical pulps for use in the production of fibrous filter media are also extremely expensive. One such commercially available chemical pulp, a mercerized southern pine, forms the basis for a superior type of conventional filter sheet, possessing a low level of fiber bonding, along with high porosity and high bulk. However, the chemical process for the manufacture of this pulp is rather complex, and the yield of pulp is quite low, about 35% based on oven dry wood. Necessarily, this pulp is very costly. Some of the other chemical pulps, particularly flash dried kraft, are somewhat less costly but are correspondingly less satisfactory on a performance basis, and their use is limited to applications in which the performance specifications of the filter media are less demanding.
In accordance with the present invention, a novel and greatly improved filter media is provided, which utilizes mechanically formed pulp in lieu of some or all of the content of high performance and other chemical pulps otherwise customarily utilized. Nevertheless, not only is there no significant loss in filter performance characteristics, but there is indeed an improvement in some of the more significant filter characteristics as a result of the substitution. Additionally, where the new filter media incorporates thermomechanically formed fibers in lieu of lower performance fibers, such as flash dried krafts, a significant performance benefit is realized.
In accordance with one of the important aspects of the invention, filter media may be produced utilizing substantial proportions of thermomechanical pulp, in place of all or part of the high performance chemical fibers. Thermomechanically produced pulp is, of course, a well known product in a general sense. It has been widely used for many years in the production of fiber board products, construction paper, newsprint, and other products, but not, insofar as is known, in the production of filter media. As one of the significant aspects of the present invention, however, it has been found that certain types of thermomechanical pulp fibers, produced under certain operating conditions, have many of the characteristics of a theoretical ideal fiber for this purpose and result in an unexpectedly superior grade of filter media.
In the thermomechanical pulping process, wood chips or other lignin-containing materials are placed under a steam atmosphere, at elevated pressure. After preheating in the pressurized steam atmosphere, the wood chips are progressively introduced between a pair of rotating refiner discs, while maintained under steam pressure. Depending upon the pressure of the steam, and the adjustment of the refiner discs, the wood chips are subjected to a controlled degree of abrasion, reducing the chips to fibrous form, after which the fibers issue from the peripheral region of the rotating disc refiner for further processing and utilization.
It has been known that, under certain pressure-temperature conditions in the disc refiner, and certain energy level utilization during the refining process, there is a heat softening or thermo plasticity of the lignin within the fiber bundles. This brings about a weakening of the so-called middle lamella, such that the bonding force between adjacent fibers is greatly reduced and the individual fibers are easily separated without excessive damage and breakage. These original discoveries date back to the early 1930s and are reflected in, for example, the Asplund U.S. Pat. No. 2,008,892. Developments in equipment and techniques for the thermomechanical manufacture of pulp have of course continued since the pioneering efforts of Asplund. Examples of more recent efforts are the Shouvlin et al. U.S. Pat. No. 3,773,610, assigned to Bauer Bros. Co., Springfield, Ohio, and the Selander et al. U.S. Pat. No. 4,221,630.
Notwithstanding the more or less continuous development effort over the last 50 years or so in the art of thermomechanical pulp production and utilization, no one, insofar as I am aware, has discovered, prior to my invention, the truly exceptional characteristics of certain types of thermomechanically produced pulp fibers for use in the manufacture of fibrous filter media.
Pursuant to the discoveries of the present invention, thermomechanically produced pulp fibers, produced under certain controlled conditions, result in a fiber which is nearly a theoretical ideal for use in fibrous filter media, far superior to ordinary chemical pulps utilized for this purpose, and indeed superior in many respects to high performance chemical pulps.
Filters made in accordance with the present invention are characterized by an exceptionally high dust capacity, high freeness and high bulk, all highly desirable characteristics of filter media. In many respects, filters produced in accordance with the present invention exceed the performance characteristics of conventional filters using high performance chemical pulps. Moreover, over a broad spectrum of filter applications, filters made in accordance with the present invention are capable of achieving superior performance.